TRAVERSE CITY, Mich. (AP) — Food supplies for fish and other
organisms are declining in some areas of the Great Lakes, particularly Lakes
Huron and Michigan, according to a newly released scientific report.

The study, based on years of data compiled by government agencies and
university researchers, found evidence of drop-offs in phytoplankton — tiny
plants essential to many food chains — since the late 1990s. A decline in
tiny invertebrates and prey fish, such as alewives and round gobies, also
was detected.

It's likely that invasive quagga and zebra mussels have played a significant
role by gobbling plankton, according to the paper, which was published
online this month in the journal BioScience. The mussels arrived in the
Great Lakes in the 1980s after being scooped into cargo ships' ballast tanks
in foreign ports and hauled across the Atlantic.

Another crucial factor is government policies that have reduced the flow of
phosphorus — a key food source for plankton — as a means of preventing
runaway algae blooms.

"As we shrink the base of the food web, it ultimately will constrain the
amount of fish we have," said David "Bo" Bunnell, lead author of the report
and a fishery biologist with the U.S. Geological Survey's Great Lakes
Science Center in Ann Arbor. The salmon population already has crashed in
Lake Huron because of steep declines in the forage fish they eat.

The study was designed to document trends in Great Lakes food webs and
determine whether the webs were influenced more by the feeding habits of top
predator fish or by developments at the lower end of the chains.

For the most part, "bottom-up" factors were found to have a greater effect.
The phosphorus shortfall in deeper waters and the mussel infestation closer
to shore were most evident in Lakes Huron and Michigan. Invertebrates that
feed prey fish were noticeably absent in both, along with Lake Ontario.

Declines in prey fish numbers were documented in Lakes Michigan, Huron and
Superior. But only in Lake Michigan was it evident that the heavy appetites
of predator fish at least partly caused the drops.

"Food isn't available for those prey fish, and it's not because we've
overstocked" bigger fish, said Tom Nalepa, a scientist with the University
of Michigan Water Center. "It's because of changes in the lower food web."

While each lake has unique characteristics, the data revealed increased
water clarity everywhere except in Lake Erie, which has been plagued by
excessive algae. Clear water can be a telltale sign of invasive mussels.

Despite the massive amounts of information used to compile the report, the
scientists said there were significant data shortages that show the need for
more intensive monitoring of Great Lakes ecosystems.

"The biggest gap in the study is that we need to do a better job of
estimating mussel populations," Nalepa said.